Adaptive filters are well known in the art and are commonly used in a number of applications. Usually, such filters are implemented in finite impulse response (FIR) form where the FIR coefficients are dynamically variable in response to detected characteristics of the filter output signal.
One illustrative application of an adaptive filter is as an equalizer for an imperfect analog anti-alias filter. In many electronic signal measurement instruments, such as Fourier-based spectrum analyzers, anti-alias filters are used to low-pass analog input signals prior to their conversion into digital form (in which form the signals are subsequently processed). This filtering is necessary so the conversion process does not yield image terms that alias back into the measurement band of interest.
While effective in attenuating signals above a cutoff frequency, analog anti-alias filters (and indeed nearly all analog filters) lack the perfectly flat passband response and phase linearity required by signal analysis instruments. Instead, signals in the passband exhibit increasing degrees of attenuation, ranging from small to substantial, with irregular phase characteristics.
One way to compensate for the non-ideal spectral response is to cascade after the anti-alias filter and the analog-to-digital converter a digital filter that has a spectral response equal but opposite that of the analog filter, emphasizing the signal components in proportion to their earlier attenuation, up to near the cutoff frequency. By this arrangement, the flatness and phase linearity of the instrument's response is maintained.
The digital filter in the foregoing instrumentation application is desirably made adaptive so that its characteristics may be changed as the characteristics of the anti-alias filter change. (The characteristics of the anti-alias filter may change either with age or temperature, or by substitution with another anti-alias filter). To update the filter coefficients, the prior art suggests stimulating the filter with a known signal and adjusting the filter coefficients to achieve the desired output signal. For example, the cascaded combination of the analog anti-alias filter and the digital equalization filter may be stimulated with a wideband signal, such as a chirp signal or a whitened noise signal, and the filter coefficients may be adjusted until the output signal has the same spectral composition as the input signal.
A significant disadvantage with the foregoing procedure, and indeed with many prior art methods of programming the coefficients of adaptive filters, is that a dedicated calibration cycle is required in which the usual input signal is interrupted and a known calibration signal is applied instead. It is far preferable to be able to program the filter coefficients dynamically while filtering the signal the filter was intended to process. While this has been achieved in limited instances in the prior art by fortuitous a priori knowledge of some characteristic of the input data signal, this is not generally the case.
In accordance with the present invention, the coefficients of an adaptive filter are updated continuously during the filter's normal operation. This is achieved by superimposing on the input data signal a known noise signal. At the filter's output, a counterpart of this known noise signal is subtracted and the resultant signal is cross correlated with past samples of the noise signal. The filter coefficients are then updated in response to these correlations so as to minimize correlation of the resultant signal with the noise signal.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.